The invention concerns a method to manufacture a solar cell in which at least one layer of a semiconductor of a second doping type is applied to a base layer of a semiconductor of a first doping type to create the solar cell, and in which a bypass diode is formed by diffusing additional suitable doping types into an unmasked site of the solar cell.
The basic problem is that series-connected solar cells require bypass diodes that suppress an arising impermissibly high reverse voltage when one or more cells are shaded.
DE 39 03 837 A1 discloses a solar cell that is built on an n-type GaAs layer. The bypass diode necessary to protect individual shaded cells when several equivalent solar cells are series-connected is located to the side of the solar cell. With this, the solar cell itself is no shaded, but the cell requires more area. This is especially problematic for aerospace application. In addition, the solar cell and bypass diode negativity affect each other in the manufacturing process.
DE 41 36 827 C2 describes two different types of solar cells. In the first type, the bypass diode is implanted on the side of the solar cell facing the incident light and accordingly shades a part of the useful surface. The second type of portrayed solar cells is characterized in that the solar cell is formed on one surface of a p-type silicon substrate, and a by-pass diode is formed on the other surface. However due to the polarity of the bypass diode, it only can be used to protect the next solar cell in the series circuit. Hence another discrete diode is required to protect the first solar cell of a series circuit. In addition, such an arrangement requires that the electrical connection between the two neighboring solar cells be made via two different electrical conductors.
EP 0 768 720 discloses the etching of a recess in the rear of the substrate of a solar cell in which, after metallizing the entire rear of the substrate and then applying a silicon adhesive, a wafer piece can be inserted that forms a diode and can be wired as a bypass diode. In this case, the solar cell and diode to not form a unit made of the same material; the solar cell and diode are only joined. The diode has to be connected to the solar cell by means of electrodes that are connected to the rear metallization of the respective or neighboring solar cell. Poorly defined conductive connections can be created between the solar cell and diode, and the interfaces between the different materials also yield additional problems.
The invention is therefore based on the problem of presenting a solar cell with an integrated bypass diode in which the diode does not shade or reduce the photoelectric surface and that is easy to manufacture, and whereby the processes for manufacturing the solar cell and diode influence each other as little as possible. In addition, the design of the electrical connections between the neighboring solar cells and between the solar cell and diode should be as simple as possible and with no interference.
The particular advantages of the solar cell according to the invention are that the two components, the solar cell and integrated bypass diode, are manufactured almost independently, and the two processes do not negatively influence each other. The effort for creating the bypass diode is very small. The photoelectric surface of the solar cell is not reduced by the bypass diode. Finally, the cell design has the advantage that a single-piece electrical conductor which is very easy to manufacture can be used as the electrical cell connector.
The invention can basically be used with a wide range of semiconductor substrates, for example: n-Si, p-Si, n-Ge, p-Ge, n-GaAs, p-GaAs, etc. The exemplary embodiment is restricted to representing the principle using an n-type Ge substrate.
An exemplary embodiment of the invention is illustrated in the drawing, wherein:
FIG. 1 is a sectional view of solar cells according to the invention.
FIG. 2 is an equivalent circuit diagram of the solar cells in FIG. 1.
FIG. 3 is a perspective view of a series circuit of two solar cells.